Processes for hydroformylating an olefin to prepare a carbonyl derivative containing one carbon atom more than the parent olefin by reacting the olefin with synthesis gas in the presence of a Group VIII metal, e.g. rhodium, in complex combination with an organic ligand, carbon monoxide also being a component of the catalyst complex, are well known in the art and of growing industrial importance. This technology is summarized, for example, in U.S. Pat. No. 3,527,809 to Pruett et al. The olefin reactant is contacted with the catalyst and the synthesis gas (a mixture of carbon monoxide and hydrogen) in the presence of a liquid reaction medium, which may or may not comprise a separate inert liquid solvent species. A gas comprising the carbon monoxide and hydrogen is typically bubbled through the liquid reaction medium which is contained in a hydroformylation reactor which can be mechanically stirred or which may be agitated solely by the action of reactant gas being bubbled therethrough. The gas, in addition to hydrogen and carbon monoxide, may also contain vapors of the reactant olefin, in a proportion which will depend upon such factors as reaction conversion rate and the volatility of the olefin.
The aldehyde hydroformylation product can be recovered from the liquid hydroformylation reaction medium in various ways, but, especially when the aldehyde is of comparatively low molecular weight, e.g., when it contains from three to about seven carbon atoms and especially when it contains from three to about five carbon atoms, it is conveniently stripped out in vapor form by distillation, evaporation, or, especially, by being stripped out of the hydroformylation reaction zone in the gases which are being bubbled through the liquid contained therein. Hershman et al. have described this technology in "I & EC Product Research and Development" 8, pp 372-375 (1969) in a discussion of the hydroformylation of propylene in a gas-sparged reactor.
In more recent years various patents and other publications have appeared directed to the use of special reaction solvents and/or special techniques for stripping the aldehyde product out of the liquid reaction medium. For example, U.S. Pat. No. 4,148,830 (Pruett et al.) recommends using high-boiling reaction by-products as the reaction solvent, with the aldehyde product being subsequently recovered from the reaction medium in a separate vaporization operation.
The employment of intensive stripping of the liquid reaction mixture not only to recover the aldehyde product but also to reduce the formation of high-boiling reaction by-products is taught in U.S. Pat. No. 4,151,209 to Paul et al., such intensive stripping serving not only to recover the product but also to reduce catalyst deactivation. The stripping can be accomplished by distillation, simple evaporation, or, especially, by the stripping action of the reaction gases being sparged through the liquid contained in the hydroformylation reactor. Any of a number of inert reaction solvents can be employed if desired, including in particular polyalkylene glycols of molecular weight of at least about 500, although the invention itself lies fundamentally in the degree of stripping which is employed and not in choice of the solvent. As the primary control for the degree of stripping, Paul et al. employ the ratio of phosphorus contained in high-boiling reaction by-products to the phosphorus contained in the ligand which is present (the reaction system with which the patentees are concerned being one which employs triorganophosphine ligand). Paul et al. supply no teachings regarding control of stripping when the ligand employed is other than a triorganophosphine, and, so long as the specified high stripping rate is employed, they are not concerned with the identity of any separately-added solvent species which may be added to the reaction system so long as it is chemically inert in the system, compatible with the reactants and catalysts, and sufficiently non-volatile that it will not be removed overhead to any great extent during the stripping operation.
There are additional factors affecting the maintenance of optimum conditions in the stripping operation. Specifically and for example overly-intensive stripping can lead to a condition in which, depending in part upon the proportions of the hydroformylation reaction vessel, the contained liquid reaction medium becomes so expanded with gas bubbles that it begins to be entrained out the top of the reactor with the exiting gases. There is also continuing need for reliable means for reducing where possible the energy requirements and the gas-handling apparatus requirements of the reaction systems as taught in the prior art and as exemplified by Paul et al.
It will also be seen that the control system of Paul et al. is essentially directed to reaction systems where phosphorus-containing ligands are employed. That is, Paul et al. teach a process control system which relies on monitoring the relative concentrations of certain phosphorus derivatives in the liquid reaction medium with the result that in nonphosphorus ligand systems it would be necessary to seek other control parameters or, at best, rely on analogies between the chemistry of organic compounds of phosphorus and those of, for example, antimony.
It is, accordingly, an object of the present invention to provide a more precise and efficient reaction control method for hydroformylation reaction systems as discussed hereinabove. It is a further object to provide a method whereby the degree of reaction product stripping can be reliably controlled without suffering the cost drawbacks of possible over-stripping and the occasional difficulties with reaction liquid entrainment sometimes experienced in the prior art processes. Other objects will be apparent from the following detailed description.